Diagnosis of the dynamic efficiency of latent heat release and the rapid intensification of supertyphoon Haiyan (2013)

Before landfall in the Philippines, Supertyphoon Haiyan in 2013 experienced a period of rapid intensification (RI) with the maximum wind speed increasing by 31ms^-1 in 24 h. A 2-km horizontal resolution full-physics cloud-resolving model is used to study the RI of Haiyan. To aid in the diagnosis of RI, we employ a dynamic efficiency factor (DEF), which is based on axisymmetric gradient wind vortex theory. The DEF measures the efficacy of convective heating at any point in generating kinetic energy. The DEF is large in the presence of strong baroclinicity near the eyewall. The simulated tropical cyclone (TC) becomes more symmetric and resembles the observations of Haiyan during the RI period. Our results highlight the nonlinear feedback process in which deep convection becomes collocated with the large DEF area near the eyewall. With the intensity increase, the eyewall convection results in significant subsidence warming at the edge of the eye rather than at the center of the eye. The overall eye warming enhances the baroclinicity near the eyewall and the system DEF in the eye-core region is increased by 33% before and after the RI onset. The vortex intensity increase during the RI period also leads to large unbalanced radial inflow in the boundary layer. The large radial inflow causes the deep convection to occur in the region of large inertial stability inside the radius of maximum wind. The deep convection further efficiently intensifies the TC. Our simulation emphasizes the importance of model resolution in simulating the internal nonlinear processes of RI.